Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1A. The device 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a conductive layer 13 or a contact layer, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. Various conductive layers comprising Mo, Ta, W, Ti, and stainless steel etc. have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use a conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. It should be noted that the structure of FIG. 1A may also be inverted if substrate is transparent. In that case light enters the device from the substrate side of the solar cell.
In a thin film solar cell employing a Group IBIIIAVIA compound absorber, the cell efficiency is a strong function of the molar ratio of IB/IIIA. If there are more than one Group IIIA materials in the composition, the relative amounts or molar ratios of these IIIA elements also affect the properties. For a Cu(In,Ga)(S,Se)2 absorber layer, for example, the efficiency of the device is a function of the molar ratio of Cu/(In+Ga). Furthermore, some of the important parameters of the cell, such as its open circuit voltage, short circuit current and fill factor vary with the molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molar ratio. In general, for good device performance Cu/(In+Ga) molar ratio is kept at around or below 1.0. As the Ga/(Ga+In) molar ratio increases, on the other hand, the optical bandgap of the absorber layer increases and therefore the open circuit voltage of the solar cell increases while the short circuit current typically may decrease. It is important for a thin film deposition process to have the capability of controlling both the molar ratio of IB/IIIA, and the molar ratios of the Group IIIA components in the composition. It should be noted that although the chemical formula is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
The first technique used to grow Cu(In,Ga)Se2 layers was the co-evaporation approach which involves evaporation of Cu, In, Ga and Se from separate evaporation boats onto a heated substrate, as the deposition rate of each component is carefully monitored and controlled.
Another technique for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is a two-stage process where at least two of the components of the Cu(In,Ga)(S,Se)2 material are first deposited onto a substrate, and then reacted with S and/or Se in a high temperature annealing process. For example, for CuInSe2 growth, thin sub-layers of Cu and In are first deposited on a substrate to form a precursor layer and then this stacked precursor layer is reacted with Se at elevated temperature. If the reaction atmosphere contains sulfur, then a CuIn(S,Se)2 layer can be grown. Addition of Ga in the precursor layer, i.e. use of a Cu/In/Ga stacked film precursor, allows the growth of a Cu(In,Ga)(S,Se)2 absorber. Other prior-art techniques include deposition of Cu—Se/In—Se, Cu—Se/Ga—Se, or Cu—Se/In—Se/Ga—Se stacks and their reaction to form the compound. Mixed precursor stacks comprising compound and elemental sub-layers, such as a Cu/In—Se stack or a Cu/In—Se/Ga—Se stack, have also been used, where In—Se and Ga—Se represent selenides of In and Ga, respectively.
Sputtering and evaporation techniques have been used in prior art approaches to deposit the sub-layers containing the Group IB and Group IIIA components of metallic precursor stacks. In the case of CuInSe2 growth, for example, Cu and In sub-layers were sequentially sputter-deposited from Cu and In targets on a substrate and then the stacked precursor film thus obtained was heated in the presence of gas containing Se at elevated temperatures as described in U.S. Pat. No. 4,798,660. More recently U.S. Pat. No. 6,048,442 disclosed a method comprising sputter-depositing a stacked precursor film comprising a Cu—Ga alloy sub-layer and an In sub-layer to form a Cu—Ga/In stack on a metallic back electrode and then reacting this precursor stack film with one of Se and S to form the compound absorber layer. U.S. Pat. No. 6,092,669 described sputtering-based equipment and method for producing such absorber layers.
One prior art method described in U.S. Pat. No. 4,581,108 utilizes a low cost electrodeposition approach for metallic precursor preparation. In this method a Cu sub-layer is first electrodeposited on a substrate. This is then followed by electrodeposition of an In sub-layer and heating of the deposited Cu/In precursor stack in a reactive atmosphere containing Se. Methods of fabricating more advanced metallic precursor stacks comprising Cu, In and Ga have recently been described in Applicant's copending U.S. application Ser. No. 11/081,308 filed Mar. 15, 2005 entitled “Technique and Apparatus for Depositing Thin Layers of Semiconductors For Solar Cell Fabrication”; U.S. application Ser. No. 11/266,013 filed Nov. 2, 2005 entitled “Technique And Apparatus For Depositing layers Of Semiconductors For Solar Cell And Module Fabrication”; U.S. application Ser. No. 11/621,101 filed Jan. 8, 2007 entitled “Precursor Containing Copper Indium, And Gallium For Selenide (Sulfide) Compound Formation”; and U.S. application Ser. No. 11/462,685 filed Aug. 6, 2006 entitled “Technique For Preparing Precursor Films And Compound Layers For Thin Film Solar Cell Fabrication”.
Irrespective of the specific approach used to grow a Cu(In,Ga)(S,Se)2 absorber film, two molar ratios mentioned before, i.e. the Cu/(In+Ga) ratio and the Ga/(Ga+In) ratio, should be closely controlled from run to run and on large area substrates. In co-evaporation techniques, this composition control is achieved through in-situ monitoring of the evaporation rates of Cu, In, Ga and Se. In two-stage techniques, which involve deposition of sub-layers to form a precursor film and then reaction of the precursor film to form the compound absorber layer, individual thicknesses of the sub-layers forming the stacked precursor film layer need to be well controlled because they determine the final stoichiometry or composition of the compound layer after the reaction step. If the sub-layers are deposited by vacuum approaches, such as evaporation and sputtering, thicknesses of the sub-layers within the precursor layer (such as thicknesses of Cu and In sub-layers) may be monitored and controlled using in-situ measurement devices such as crystal oscillators, or sensors that sense the material deposition flux. In the prior art electroplating techniques, on the other hand, thickness of individual sub-layers such as the Cu sub-layer, In sub-layer, and/or Ga sub-layer have been controlled through control of the charge passed during deposition of the sub-layer, i.e. through control of the deposition current density and the deposition time. However, during processing, it is possible that deposition rates change. For example, in electrodeposition approach the deposition rates may vary due to reasons such as a change in the electrodeposition efficiency, age of the deposition bath, accumulation of byproducts in the bath, change in organic or inorganic additive concentrations and/or activities etc. in the electrolyte. Since solar cell efficiency is a strong function of the molar ratios of the elements within the deposited precursors, new techniques that assure excellent control of these ratios are needed.